1. Field of the Invention
The present invention relates to a valve gear including rocker arms having roller bearings at contact portions thereof with valve gear cams, and an overhead cam type four-cycle engine having such a valve gear. Further, the present invention relates to a vehicle such as a motorcycle including a four-cycle engine and such a valve gear.
2. Description of the Related Art
So-called SOHC (Single Overhead Camshaft) type four-cycle engines are known and include a single camshaft that drives exhaust valves and intake valves. This type of four-cycle engine includes an exhaust rocker arm, which transmits movements of an exhaust cam to exhaust valves, and an intake rocker arm, which transmits movements of an intake cam to intake valves.
The exhaust rocker arm and the intake rocker arm, respectively, are swingably supported on rocker shafts. The rocker shafts are arranged in parallel to each other with a camshaft therebetween. Therefore, the exhaust rocker arm is located across the rocker shaft from the exhaust cam and extends toward the exhaust valves, and the intake rocker arm is located across the rocker shaft from the intake cam and extends toward the intake valves.
JP-B-07-068892 discloses a valve gear for four-cycle engines, in which roller bearings are incorporated into an exhaust rocker arm and an intake rocker arm, respectively. The roller bearings come into rolling contact with an exhaust cam and an intake cam to thereby restrict a frictional resistance to a small amount, the frictional resistance being generated at contact portions between the exhaust rocker arm and the exhaust cam and at contact portions between the intake rocker arm and the intake cam.
In this conventional valve gear, when rotation of a camshaft causes the roller bearing of the intake rocker arm to contact a cam nose of the intake cam as it moves from a base circle thereof, the cam nose pushes up the roller bearing. As a result, the intake rocker arm swings on a rocker shaft to push intake valves in an opening direction.
The rocker shaft, which supports the intake rocker arm, is positioned rearwardly of a centerline, which passes through a center of the camshaft extending axially of a cylinder, in a direction of rotation of the camshaft. Therefore, when the cam nose pushes up the roller bearing of the intake rocker arm, the rocker shaft, which supports the intake rocker arm, is not moved in a direction in which the roller bearing is pushed up.
In other words, in a process in which the cam nose of the intake cam pushes up the roller bearing, a force exerted on a contact portion, at which the cam nose and the roller bearing contact with each other, acts in a direction intersecting a line which connects a center of rotation of the roller bearing and a center of the rocker shaft. Accordingly, a force with which the cam nose pushes up the roller bearing acts as a force by which the intake rocker arm is caused to swing on the rocker shaft, so that any undesired force will not be applied to the intake rocker arm.
On the other hand, the rocker shaft, which supports the exhaust rocker arm, is positioned forwardly of a centerline, which passes through the center of the camshaft in the direction of rotation of the camshaft. Therefore, in a process in which the cam nose of the exhaust cam pushes up the roller bearing of the exhaust rocker arm, that rocker shaft which supports the exhaust rocker arm is moved in a direction in which the roller bearing is pushed up. Accordingly, a force exerted on a contact portion at which the cam nose and the roller bearing contact each other, acts along a line which connects a center of rotation of the roller bearing and a center of the rocker shaft.
As a result, a force with which the cam nose pushes up the roller bearing, acts as a force which causes the exhaust rocker shaft to buckle, so that a load being applied on and borne by the exhaust rocker arm is increased.
Accordingly, it is necessary to take various measures to enable the exhaust rocker arm to handle such a buckling load. As a result, the exhaust rocker arm becomes disadvantageously heavy and large in size.